1. Field of the Invention
The invention relates generally to the field of biotechnology. More specifically, the invention relates to modified dicamba monooxygenase enzymes capable of conferring tolerance to the herbicide dicamba in transgenic organisms.
2. Description of the Related Art
Methods for production of field crops, such as corn, soybeans and cotton, have changed dramatically during the past decade due to the introduction of traits such as insect-resistance and herbicide tolerance through use of plant genetic engineering techniques. These changes have resulted in greater productivity per hectare, decreased production costs, greater flexibility and efficiencies in production regimes, decreased pesticide use, and, in the case of insect-resistant cotton, improved farmer health. Transgenic crops have thus gained widespread adoption and are now grown on millions of acres across the world. However, for transgenic crops to continue to be competitive in the market place, new value-added traits will be required.
Although new traits improving the quantity and quality of agricultural and horticultural crops have appeared and will continue to appear at an increasing rate in years to come, demand exists for traits that improve methods for the production of food, feed and other products. For example, while transgenic plants tolerant to treatments with the herbicides glyphosate, bromoxynil, sulphonylureas and other herbicides are presently available, there are gaps in the spectrum of weeds controlled and treatment options that can be addressed through development of additional herbicide-tolerant crops. Moreover, the appearance of weeds resistant to the herbicides noted above, while generally localized and variably contained, impose the need for supplemental or alternative weed control measures.
While transgenic herbicide tolerance has proven valuable in a commercial setting, plants tolerant to other herbicides are therefore needed to avoid over reliance on any single herbicide and to increase options for managing difficult to control weed species. Of particular need is the development of herbicide tolerance for herbicides that are both environmentally friendly and highly effective for controlling weeds. Dicamba is one such example of an effective and environmentally friendly herbicide that has been used by farmers for more than 40 years. Dicamba is especially useful for the control of annual and perennial broadleaf weeds and several grassy weeds in corn, sorghum, small grains, pasture, hay, rangeland, sugarcane, asparagus, turf, and grass seed crops (Crop Protection Reference, 1995). Unfortunately, dicamba can injure many commercial crops and dicot plants such as soybeans, cotton, peas, potatoes, sunflowers, and canola, which are particularly sensitive to even low levels of the herbicide. Despite this, dicamba is highly effective in controlling weed growth and thus an important tool in agriculture.
Recently, a gene encoding dicamba monooxygenase (DMO) was isolated from Pseudomonas maltophilia that confers tolerance to dicamba (U.S. Pat. No. 7,022,896). DMO is involved in conversion of herbicidal dicamba (3,6-dichloro-o-anisic acid) to a non-toxic 3,6-dichlorosalicylic acid. This gene is disclosed in U.S. Pat. No. 7,022,896 as providing tolerance to dicamba in plants expressing the DMO gene. However, the development of variants of this gene would be of great benefit. Such variants could potentially have altered expression efficiency under specific environmental conditions. In this manner, a variant could be selected that is optimized for a specific environment in which it is intended to be used, and may exhibit particularly beneficial kinetic features. The variant in particular may exhibit maximum efficiency at different temperatures or pH conditions, and thus could be selected for a particular crop species depending upon intracellular conditions and/or the anticipated crop growing conditions.